Background: Circulating endothelial cells (CECs) have been studied as a biomarker for tumour progression and monitoring therapeutic effects. The CellSearch system is a semi-automated system that allows standardised analysis of CECs. This study assessed the clinical implications of CECs determined by the CellSearch system in breast cancer patients. Methods: Seventy-six consecutive breast cancer patients (53 operable and 23 metastatic or recurrent) were enrolled for the study. Thirty-five patients with operable breast cancer received preoperative chemotherapy with a regimen based on anthracycline and/or taxane. CECs are defined as CD146+CD105+CD45)DAPI+ cells in the system. CD34 expression was examined using the additional channel in the system. Results: A majority (4539 of 5183 cells, 88%) of CECs from patients with operable breast cancer were CD34-positive. Triple-negative cancers showed higher baseline CEC and CD34+CEC counts than the other types (P = 0.0387 and 0.0377, respectively). Low baseline CEC and CD34+CEC counts, and a low CD34 positive rate were associated with pathological complete response (pCR) of preoperative chemotherapy in patients with primary breast cancer (P = 0.046, 0.027 and 0.01, respectively). In multivariate analyses, the CD34 positive rate was significant for pCR (P = 0.021). During preoperative chemotherapy, CEC and CD34+CEC counts before each cycle of chemotherapy increased with taxane-based regimens (P = 0.0018 and 0.0008, respectively) but not with anthracycline-based regimens. Conclusions: Baseline CEC, in particular CD34+CEC, counts and the CD34 positive rate might be useful for the prediction of treatment response of preoperative chemotherapy in patients with operable breast cancer.